1. Field of the Invention
The present invention relates to a thermally protective, flame retardant fabric and, more particularly, to a lightweight fabric providing protection from heat, flame, and electrical arc that is suitable for use in a wide range of products. Applications of the fabric include protective garments, articles of furniture, vehicle components, building components, electrical components, decorative components, appliances, and containers.
2. Description of the Related Art
Thermal protective fabrics are known in the art. In one application, apparel made from these fabrics protects users in a range of hazardous environments. Thermally protective fabrics typically provide a combination of thermal insulation properties and heat reflection and/or absorption properties. This combination of properties may reduce or eliminate heat-related and burn-related injuries.
There are several qualities a fabric may possess in order to be a good thermal insulator. One quality is the ability of the fabric to trap air. A fabric with good air-trapping features may be formed by constructing the fabric with fibers, such as cotton or wool, that are themselves good insulators. Such a fabric may also be formed by constructing the fabric in such a way that it provides interstices or layers in which air or other gases can collect. One example of such a fabric is a needlepunched, nonwoven material. Needlepunched, nonwoven fabrics are manufactured by overlapping carded layers of fiber and then entangling them by penetrating the layers with rigid needles. The result is a soft, lofty fabric with many pockets for air collection.
Heat reflection and/or absorption properties in a fabric may be provided by a finish, such as a coating, that can reflect and/or absorb heat. Conventional thermally protective fabrics have used coatings made from metallized compounds, including aluminum or titanium, to reflect the heat energy. However, these finishes are typically stiff, difficult to apply, and expensive.
Coatings used to absorb heat have been formed from one or more intumescent compounds. Intumescent compounds are compounds that react on contact to flame by charring and swelling. The layers of char that are formed may fill with nonflammable gas created in the intumescent reaction and, thus, provide more layers of insulation. Intumescent compounds have typically been used in building materials and paints to prevent the spread of fire and structural damage. These compounds, however, have been used with only limited success in the field of textiles.
The degree of thermal protection provided by a fabric is measured with an industry standard test. The NFPA 1971 Standard on Protective Ensemble for Structural Fire Fighting, Section 6-10 describes a Thermal Protective Performance (TPP) test for predicting time to second-degree burn when exposed to convective/radiant energy for a short duration.
In the test, the thermal resistance of three 6″×6″ samples is averaged using a CSI Thermal Protective Performance Tester. Heat exposure is provided by a combination of a largely convective heat source provided by two laboratory burners and a radiant source provided by a bank of quartz tubes. The gas burners are set at 45 degrees to vertical so that the flames converge at a point directly beneath the sample and burn 98% pure methane at a flow rate of 135 units on the CST apparatus. The quartz tubes are adjusted to 48% on the instrument scale. The instrument is calibrated to insure the delivery of an exposure averaging 2.0 cal/cm2 sec.
The fabric sample to be tested is mounted in a sample holder positioned above the heat source. The heat transfer through the fabric is measured by a calorimeter that is placed above the fabric sample, either in direct contact with the sample or suspended above the sample by means of a standard spacer. Test results for these two types of tests are reported as “contact” or “spaced” results, respectively.
During the test, a computer utilizing specially designed data acquisition software accurately records the rise in temperature of the calorimeter. The rate of temperature rise (i.e., the slope of the temperature vs. time trace) is used in conjunction with the calorimeter constants to compute the heat flux received. A square wave exposure sequence is used so that results can be related to the values obtained in a Stoll curve. A human tissue tolerance overlay, obtained by integration of the Stoll curve with respect to time, is used to determine tolerance times to second-degree burns. The TPP rating is calculated as the product of exposure energy heat flux and time to second-degree burn.
Table 1 lists the TPP test results for several conventional thermally protective fabrics.
TABLE 1TPP Performance of Conventional FabricsTPPTPPWeightTPPEfficiency1TPPEfficiencyFabric(osy)(contact)(contact)(spaced)2(spaced)NOMEX4.54.81.111.82.6IIIA6.15.10.813.42.27.516.12.1INDURA6.07.31.28.46.60.89.41.110.07.10.711.11.1BANWEAR8.69.41.211.512.71.1FIREWEAR5.68.41.59.511.01.21Efficiency is defined as TPP/weight.2¼″ spacer placed between the sample and the sensor
The highest TPP value seen in Table 1 is 16.1 on 7.5 ounces per square yard (osy) NOMEX IIIA during a spaced test, meaning that a ¼″ spacer was placed between the sample and the sensor. The efficiency (spaced) of this weight fabric is therefore 2.1. As used herein, the term “efficiency” means TPP/weight. Note that the efficiency (contact) of this same fabric at lower weights is significantly reduced to 1.1 for the 4.5 osy product and 0.8 for the 6.1 osy product. A fabric that can produce TPP values in these ranges at lower weights is therefore a more efficient insulator and would offer users a lighter weight alternative without sacrificing protection.
Most conventional fabrics in the thermal protection market are designed for extended use for periods of one year or more. These fabrics must therefore be durable enough to withstand continual use, possibly in an industrial environment. In the case of garments, such use may include repeated laundering and repeated wear. In addition, thermally protective fabrics must remain flame retardant and thermally protective during the period of use. In order to achieve this durability, conventional fabrics have increased thickness and weight, which limit their versatility.
In one illustrative example, conventional fabrics may be used to make thermally protective garments. The most prevalent fabrics in the thermally protective garment market are aramids and flame retardant cotton. Most high performance thermally protective fabrics are aramids, such as NOMEX IIIA made by Dupont. For example, these fabrics dominate the fire department wear market. Flame retardant cotton, on the other hand, is used more extensively in general industrial use. This is due primarily to the more favorable hand (i.e., texture) and comfort of flame retardant cotton, and the significantly higher costs associated with aramid fabrics.
This pattern of usage indicates industry's concern over the capital expense associated with thermal protective apparel programs. Aramid fabrics are generally considered superior to flame retardant cotton in terms of durability, launderability, and thermal performance, yet the price and comfort associated with flame retardant cotton make it a desirable alternative. The market strength of aramids in a particular industry increases as the risk of exposure to fire increases.
Conventional aramid fabrics include NOMEX IIIA from Dupont, PBI from Hoechst Celanese, and KERMEL from Rhone-Poulenc Fibers. These fabrics are available in a variety of weights and may be blended with other fibers to reduce cost. Common uses for these fabrics include fireman's bunker gear, fire entry suits, apparel for utility workers, and apparel for some industrial applications.
Conventional flame retardant cotton fabrics and blended fabrics include INDURA from Westex, Inc., FIREWEAR from Springfield, and BANWEAR from ITEX, Inc. Other fabrics include BASOFIL from BASF, made from a melamine fiber, and FR VISCOSE from Lenzing Fibers, made from a permanently flame retardant viscose. The above fabrics are available in a variety of weights. Common uses include flame retardant apparel, such as coveralls, shirts, and pants for general industry, apparel for utility workers, and fireman's stationwear.
The above fabrics have been used to produce a variety of durable thermally protective products suitable for extended use in their respective industries. However, each of these products has deficiencies, such as weight, comfort, and cost. These and other deficiencies of conventional thermally protective fabrics have limited and, in some cases, precluded their use in a variety of applications other than garments, such as articles of furniture, vehicle seats, vehicle bodies, electrical products, building components, and flame blocking components.
There is currently a need for lightweight, low cost, fabrics that provide a high degree of protection from heat caused by flame and electrical arc, for example.